1. Field of the Invention
This invention relates generally to a posterior cervical implant assembly for holding adjacent vertebral bones immobilized via fixation to the lateral masses. More particularly, this invention relates to a novel assembly of bone screws and plates for use in surgical procedures for stabilizing the relative motion of, or permanently immobilizing, vertebral bodies in the cervical spine, wherein the screws form a polyaxial coupling of the plate to the bone, and which coupling provides a locking of the screws to the plate through a wide range of entrance angulation.
2. Description of the Prior Art
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. For the purposes of this disclosure, however, the word spine shall refer only to the cervical region.
Referring now to FIGS. 1 and 2, a typical cervical body is shown in a top view, and a pair of cervical bones and the interspecing disc are shown in a side view. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 includes a rearwardly and downwardly extending portion called the spinous process 16, and laterally extending bulk structures, correspondingly referred to as the lateral masses 14. The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. The lateral masses 14 comprise bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. The most flexible of all the regions of the spinal column is the cervical spine.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease are a few of the causes which can result in spinal pathologies for which permanent immobilization of multiple vertebrae may be necessary. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies.
The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. The use of screw plate assemblies (lateral mass plates) for stabilization and immobilization of the cervical spine is, however, common. A lateral mass plate is a narrow elongate plate having a series of spaced through holes which screws may be inserted to fix the plate to the bones. A pair of lateral mass plates are placed across the lateral posterior surfaces of a set of sequential cervical bones and are secured to the bone with screws, thereby preventing the bones from moving relative to one another in either the vertical or horizontal planes.
Because the spine is routinely subject to high loads which cycle during movement, one of the primary concerns of physicians performing spinal implantation surgeries, as well as of the patients in whom the implants are placed, is the risk of screw pull-out. Screw pull-out occurs when the cylindrical portion of the bone which surrounds the inserted screw fails. A bone screw which is implanted perpendicular to the plate is particularly weak because the region of the bone which must fail for pull-out to occur is only as large as the outer diameter of the screw threads. It has been found that for pull-out to occur for screws which are inserted into the bone at an angle with respect to the plate, the amount of bone which must fail increases substantially as compared with screws which are implanted perpendicularly with respect to the plate. It has, therefore, been an object of those in the art to provide a pair of lateral mass screw plate assembly which permits the screws to be entered into the lateral masses at angles other than 90 degrees.
An additional concern with screws being implanted in the lateral masses of the cervical spine, is that their are sensitive and/or important structures adjacent to the masses, which, because of their proximity to the implant, may be damaged by insertion or dislocation of screws. In the cervical spine, the vertebral arteries are disposed medially beneath the lateral masses and comprise critical structures which cannot be compromised. In addition, the facet joints which provide natural coupling of sequential bones together must also be avoided it possible. Avoidance of these bodies has been a critical and ongoing concern with respect to posterior screw insertion. Lateral mass plates of the prior art have provided little in the way of reasonable or practical solutions for ensuring proper screw insertion.
Lateral mass screw plate assemblies necessarily include a plurality of screws which are inserted through a single plate. If a single screw loosens with respect to the surrounding bone into which it has been inserted, loss of fixation occurs and possible neurological repercussions may result. Unfortunately, as the screws of the prior art have included no means to prevent screws from "backing out" of the holes once they have loosened. It has, therefore, been a goal of those in the art to minimize the risk of screw pull-out which happens by bone failure followed by cyclical force induced back-out.
While the preceding discussion has specifically focused on cervical lateral mass screw plate systems and the concerns associated therewith, similar concerns apply to the art of posterior vertebral immobilization viw screw plate systems which are presently available in the thoracic and lumbar spine as well.
It is therefore, a principal object of the present invention to provide a new and novel cervical lateral mass screw plate design having a polyaxial coupling of the screw to the plate, whereby a single plate is compatible with a wide range of screw-in angles.
It is also, therefore, an object of the present invention to provide a screw plate design which provides the surgeon with the greatest freedom to choose the most desirable angle to direct the bone screw, so as to avoid critical structures in the cervical spine.
It is also an object of the present invention to provide a spinal insert assembly which is more sturdy and more versatile than previous designs.
It is also a principal object of the present invention to provide an orthopedic screw plate assembly which has a simple and effective locking mechanism for locking the bone screw to the plate.
It is also, therefore, an object of the present invention to provide a lateral mass screw plate assembly which resists screw pull-out failure by preventing screw back-out resulting from screw-bone failure coupled with kinesthetic force cycling.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.